A reactor core with fuel assemblies as described above is immersed into water which serves both as coolant and as neutron moderator. The space between the fuel assemblies is filled with flowing water for neutron moderation. The distance between two fuel assemblies is referred to as gap width.
In addition to the water flowing along the fuel rods and the water being in contact with the fuel rods, the reactor core is traversed by a by-pass flow which is located at the above-mentioned gaps formed between the fuel assemblies. The major part of the water, typically 90%, is admitted into the fuel assemblies and allowed to boil therein. The remainder is discharged through the by-pass holes and is maintained in a nonboiling state. In certain cases, the fuel assemblies are provided with internal water channels. The water in these channels is not allowed to boil.
To obtain a specified quantity of by-pass flow outside the fuel assemblies, that is, in the gaps, the transition section of the fuel assemblies are provided with so-called by-pass holes which allow the passage of the required quantity of water outside the fuel assemblies.
Swedish printed patent application 424 236 discloses an example of a fuel assembly provided with continuous by-pass holes in the transition section. These holes are arranged at the center of the side surface of the fuel assembly and exactly opposite to each other, whereby the by-pass flow from one of the holes is always directed against the control rod.
U.S. Pat. No. 5,697,376 discloses another example of a fuel assembly provided with continuous by-pass holes in the transition section. These two holes are arranged at the center of the side surface of the fuel assembly with an approximately 90.degree. angular adjustment. This location of the holes means that the fuel assembly can be arranged in one position only to prevent the by-pass flow from one of the holes from being directed against the control rod.
In certain cases there is a need to turn the fuel assembly through an angle of 90.degree., 180.degree. or 270.degree. around its longitudinal axis. One such case is when bending the fuel channel of the fuel assembly. Bending of the fuel channel may occur after some time of use and means that the fuel channel changes its shape by bending. Bending of the fuel channel may, in certain cases, constitute a problem in symmetrical core lattices. In symmetrical core lattices, the control rod gaps, that is, the gaps into which the control rods are inserted, and the narrow gaps, that is, the gaps into which the control rods are not inserted, have the same gap width. In symmetrical lattices, the fuel channel is bent in a random direction when being irradiated with fast neutrons. The fast neutrons entail release of residual stresses, whereby the structure of the material is changed such that the fuel channel is extended. When the different sides of the fuel channel are extended to different degrees, the fuel channel becomes deflected towards the direction where the largest extension has taken place.
In those cases where the fuel channel is bent in a direction towards the control rod in a supercell, the bending can be of such a magnitude that the control rod when being inserted into the core, for power control, adheres thereto. In those cases where this bending takes place in a direction towards the control rod, thus, the box should be turned 90.degree., 180.degree. or 270.degree. around its longitudinal axis to eliminate the risk of the bent fuel channel preventing the insertion of the control rod. When being bent, the fuel channel has a certain memory effect such that, if a fuel channel starts bending towards the control rod and the channel is turned such that it bends in a direction away from the control rod, the fuel channel will continue to bend in this direction, that is, away from the control rod. The memory effect is due to the fact that the gap towards which the fuel channel has started bending becomes smallest and is hence irradiated by more fast neutrons, whereby the bending continues towards this smallest gap.
During refuellings, the fuel assemblies are picked out of the core and its fuel channels are inspected with respect to bending. In those cases where bending in a direction towards the control rod has been determined, the fuel assembly is turned 90.degree., 180.degree. or 270.degree. around its longitudinal axis. The problem is then that the holes which, according to the prior art, are provided centrally on the two sides of the fuel assembly, facing away from the control rod, are arranged to direct the by-pass flow against the control rod. A by-pass flow directed against the control rod may cause the rod to vibrate and hence wear out holes in the fuel channel, which may lead to a reduction of the safety margin with respect to so-called dryout since less water is passed into the fuel assembly when the pressure is lower outside thereof.